Flexible printed circuits have gained widespread use in applications where flexing is a functional requirement, such as where flexing facilitates assembly and disassembly, or other dimensional variations need to be accommodated. In general, flexible circuits are formed by bonding a metal foil, such as a copper foil, onto a flexible insulating film or substrate such as a polyester resin film. A circuit is formed in the metal foil by conventional etching techniques.
Flexible printed circuits may be covered with an additional layer of a similar insulating film or substrate to provide additional insulation and to protect the circuit pattern from contamination, corrosion, and mechanical damage. The cover film may also be necessary for the formation of a symmetrical layered structure and/or a solder stop mask.
Conventional cover films may be made of polyester or polyimide polymers. Holes may be introduced into the film so that solder "eyes" to the flexible circuit boards remain clear. The cover film may be bonded to the flexible circuit board with a heat-activated adhesive coating under pressure and heat.
A disadvantage of using a heat-activated adhesive to effect such bonding is that such adhesives flow when subjected to pressure and heat. Thus, any holes that have been introduced into the cover film may be occluded by the flowing adhesive, reducing the available solder surface beneath them. The extent of adhesive flow is difficult to control, since it depends, among other things, its degree of polymerization and the proportion of volatile components in the adhesive.
One approach to control the problem of flowing adhesives has been to employ a non-flowing adhesive. However, this method is also associated with its own problems. For example, the non-flowing adhesive can trap air that cannot be completely displaced. The trapped air leaves voids that may permit solder to migrate below the cover film during hot-tinning or soldering.
Another problem associated with flexible printed circuit boards is that heat generated by a current flowing through a circuit can cause the substrate, the insulating film, or the adhesive, to be ignited. Thus, flexible printed circuit boards have a potential to cause fire in instruments and devices utilizing such circuit boards. A proposed solution to this problem is to include flame retardants in the cover film adhesive.
U.S. Pat. No. 4,075,420 is directed to a cover layer for flexible circuit boards. The disclosed cover layer is formed by a three-layer laminate, which includes an insulation film layer, a thermosetting adhesive layer and a phenolic resin layer. The phenolic resin layer connects the cover film to a flexible circuit board.
U.S. Pat. No. 4,311,749 is directed to a flame-proof flexible printed circuit board which includes a flexible resin film and a copper foil electric circuit, bonded together with an adhesive. The flexible resin film is treated to provide a surface tension of 36 dyne/cm, or more, and a resin film containing a flame retardant is applied to the treated surface of the flexible resin film.